Single-screw compressors (see FIG. 9) having a compression mechanism which compresses a refrigerant by rotational movement of a screw rotor have been known. In this single-screw compressor (hereinafter referred to as a “screw compressor”) (100), a gate rotor (150) meshes with a screw rotor (140) rotating in a cylinder wall (131) of a casing (130), through an opening in the cylinder wall (131), thereby forming a compression chamber (123). One end of the screw rotor (140) (i.e., the left end of the drawing) is a suction side, and the other end (i.e., the right end of the drawing) is a discharge side. When the suction side of the screw rotor (140) is closed by the gate rotor (150), a compression chamber (123) in which a low-pressure gas is sealed in a helical groove of the screw rotor (140) is formed. From there, the screw rotor (140) is further rotated, making the compression chamber (123) small, until the compression chamber (123) moves to the discharge side and communicates with a discharge opening (125). At this time, the high-pressure gas is released to the discharge side of the casing (130).
In the screw compressor (100), it is suggested to provide a slide valve (104) which moves along the axial direction of the screw rotor (140), as a variable VI mechanism (i.e., a volume ratio adjusting mechanism) (103) for adjusting a ratio between the suction volume and the discharge volume (i.e., a volume ratio:VI) (see, for example, Japanese Patent No. 4147891). The slide valve (104) is moved along the axial direction of the screw rotor (140) to change the discharge volume by changing the position from which the high-pressure gas starts to be discharged (i.e., completion of compression), thereby changing the ratio of the discharge volume to the suction volume.
The screw compressor (100) is configured to change the rotational speed of an electric motor (not shown) by controlling an inverter, thereby controlling the operating capacity. The operating capacity (i.e., the amount of refrigerant discharged per unit time) is controlled according to a load on the utilization side of the refrigerant circuit. Here, the slide valve (104) of the variable VI mechanism (103) is controlled to obtain a volume ratio (i.e., compression ratio) which can lead to optimal compression efficiency, with respect to the operating capacity controlled according to the load. Thus, the slide valve (104) moves along the axial direction of the screw rotor (140) according to the operating capacity which varies depending on whether the operation state is a rated load state (100% load) or a part load state (see FIGS. 10(A) and 10(B)).
A discharge side end surface (104a) of the slide valve (104) is preferably in the shape corresponding to a screw land (142) (i.e., the surface along the raised portion between the helical grooves of the screw rotor (140)) to which the discharge side end surface (104a) faces to reduce the pressure loss of the discharged fluid. However, the angle and the width of the screw land (142) are not uniform from the suction side to the discharge side. Therefore, to efficiently reduce the pressure loss of the discharged fluid at the time of a rated load operation (i.e., the largest operating capacity), the discharge side end surface (104a) of the slide valve (104) has been formed into a shape which corresponds to the inclination of the screw land (142) facing the discharge side end surface (104a) at the time of the rated load operation as shown in FIG. 10(A).